/*
 * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package java.util;

import java.util.function.Consumer;

/**
 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
 * The elements of the priority queue are ordered according to their
 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
 * provided at queue construction time, depending on which constructor is
 * used.  A priority queue does not permit {@code null} elements.
 * A priority queue relying on natural ordering also does not permit
 * insertion of non-comparable objects (doing so may result in
 * {@code ClassCastException}).
 *
 * <p>The <em>head</em> of this queue is the <em>least</em> element
 * with respect to the specified ordering.  If multiple elements are
 * tied for least value, the head is one of those elements -- ties are
 * broken arbitrarily.  The queue retrieval operations {@code poll},
 * {@code remove}, {@code peek}, and {@code element} access the
 * element at the head of the queue.
 *
 * <p>A priority queue is unbounded, but has an internal
 * <i>capacity</i> governing the size of an array used to store the
 * elements on the queue.  It is always at least as large as the queue
 * size.  As elements are added to a priority queue, its capacity
 * grows automatically.  The details of the growth policy are not
 * specified.
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.  The Iterator provided in method {@link
 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
 * the priority queue in any particular order. If you need ordered
 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * Multiple threads should not access a {@code PriorityQueue}
 * instance concurrently if any of the threads modifies the queue.
 * Instead, use the thread-safe {@link
 * java.util.concurrent.PriorityBlockingQueue} class.
 *
 * <p>Implementation note: this implementation provides
 * O(log(n)) time for the enqueuing and dequeuing methods
 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
 * linear time for the {@code remove(Object)} and {@code contains(Object)}
 * methods; and constant time for the retrieval methods
 * ({@code peek}, {@code element}, and {@code size}).
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this collection
 * @author Josh Bloch, Doug Lea
 * @since 1.5
 */
public class PriorityQueue<E> extends AbstractQueue<E>
    implements java.io.Serializable {

  private static final long serialVersionUID = -7720805057305804111L;

  private static final int DEFAULT_INITIAL_CAPACITY = 11;

  /**
   * Priority queue represented as a balanced binary heap: the two
   * children of queue[n] are queue[2*n+1] and queue[2*(n+1)].  The
   * priority queue is ordered by comparator, or by the elements'
   * natural ordering, if comparator is null: For each node n in the
   * heap and each descendant d of n, n <= d.  The element with the
   * lowest value is in queue[0], assuming the queue is nonempty.
   */
  transient Object[] queue; // non-private to simplify nested class access

  /**
   * The number of elements in the priority queue.
   */
  private int size = 0;

  /**
   * The comparator, or null if priority queue uses elements'
   * natural ordering.
   */
  private final Comparator<? super E> comparator;

  /**
   * The number of times this priority queue has been
   * <i>structurally modified</i>.  See AbstractList for gory details.
   */
  transient int modCount = 0; // non-private to simplify nested class access

  /**
   * Creates a {@code PriorityQueue} with the default initial
   * capacity (11) that orders its elements according to their
   * {@linkplain Comparable natural ordering}.
   */
  public PriorityQueue() {
    this(DEFAULT_INITIAL_CAPACITY, null);
  }

  /**
   * Creates a {@code PriorityQueue} with the specified initial
   * capacity that orders its elements according to their
   * {@linkplain Comparable natural ordering}.
   *
   * @param initialCapacity the initial capacity for this priority queue
   * @throws IllegalArgumentException if {@code initialCapacity} is less than 1
   */
  public PriorityQueue(int initialCapacity) {
    this(initialCapacity, null);
  }

  /**
   * Creates a {@code PriorityQueue} with the default initial capacity and
   * whose elements are ordered according to the specified comparator.
   *
   * @param comparator the comparator that will be used to order this priority queue.  If {@code
   * null}, the {@linkplain Comparable natural ordering} of the elements will be used.
   * @since 1.8
   */
  public PriorityQueue(Comparator<? super E> comparator) {
    this(DEFAULT_INITIAL_CAPACITY, comparator);
  }

  /**
   * Creates a {@code PriorityQueue} with the specified initial capacity
   * that orders its elements according to the specified comparator.
   *
   * @param initialCapacity the initial capacity for this priority queue
   * @param comparator the comparator that will be used to order this priority queue.  If {@code
   * null}, the {@linkplain Comparable natural ordering} of the elements will be used.
   * @throws IllegalArgumentException if {@code initialCapacity} is less than 1
   */
  public PriorityQueue(int initialCapacity,
      Comparator<? super E> comparator) {
    // Note: This restriction of at least one is not actually needed,
    // but continues for 1.5 compatibility
    if (initialCapacity < 1) {
      throw new IllegalArgumentException();
    }
    this.queue = new Object[initialCapacity];
    this.comparator = comparator;
  }

  /**
   * Creates a {@code PriorityQueue} containing the elements in the
   * specified collection.  If the specified collection is an instance of
   * a {@link SortedSet} or is another {@code PriorityQueue}, this
   * priority queue will be ordered according to the same ordering.
   * Otherwise, this priority queue will be ordered according to the
   * {@linkplain Comparable natural ordering} of its elements.
   *
   * @param c the collection whose elements are to be placed into this priority queue
   * @throws ClassCastException if elements of the specified collection cannot be compared to one
   * another according to the priority queue's ordering
   * @throws NullPointerException if the specified collection or any of its elements are null
   */
  @SuppressWarnings("unchecked")
  public PriorityQueue(Collection<? extends E> c) {
    if (c instanceof SortedSet<?>) {
      SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
      this.comparator = (Comparator<? super E>) ss.comparator();
      initElementsFromCollection(ss);
    } else if (c instanceof PriorityQueue<?>) {
      PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
      this.comparator = (Comparator<? super E>) pq.comparator();
      initFromPriorityQueue(pq);
    } else {
      this.comparator = null;
      initFromCollection(c);
    }
  }

  /**
   * Creates a {@code PriorityQueue} containing the elements in the
   * specified priority queue.  This priority queue will be
   * ordered according to the same ordering as the given priority
   * queue.
   *
   * @param c the priority queue whose elements are to be placed into this priority queue
   * @throws ClassCastException if elements of {@code c} cannot be compared to one another according
   * to {@code c}'s ordering
   * @throws NullPointerException if the specified priority queue or any of its elements are null
   */
  @SuppressWarnings("unchecked")
  public PriorityQueue(PriorityQueue<? extends E> c) {
    this.comparator = (Comparator<? super E>) c.comparator();
    initFromPriorityQueue(c);
  }

  /**
   * Creates a {@code PriorityQueue} containing the elements in the
   * specified sorted set.   This priority queue will be ordered
   * according to the same ordering as the given sorted set.
   *
   * @param c the sorted set whose elements are to be placed into this priority queue
   * @throws ClassCastException if elements of the specified sorted set cannot be compared to one
   * another according to the sorted set's ordering
   * @throws NullPointerException if the specified sorted set or any of its elements are null
   */
  @SuppressWarnings("unchecked")
  public PriorityQueue(SortedSet<? extends E> c) {
    this.comparator = (Comparator<? super E>) c.comparator();
    initElementsFromCollection(c);
  }

  private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
    if (c.getClass() == PriorityQueue.class) {
      this.queue = c.toArray();
      this.size = c.size();
    } else {
      initFromCollection(c);
    }
  }

  private void initElementsFromCollection(Collection<? extends E> c) {
    Object[] a = c.toArray();
    // If c.toArray incorrectly doesn't return Object[], copy it.
    if (a.getClass() != Object[].class) {
      a = Arrays.copyOf(a, a.length, Object[].class);
    }
    int len = a.length;
    if (len == 1 || this.comparator != null) {
      for (int i = 0; i < len; i++) {
        if (a[i] == null) {
          throw new NullPointerException();
        }
      }
    }
    this.queue = a;
    this.size = a.length;
  }

  /**
   * Initializes queue array with elements from the given Collection.
   *
   * @param c the collection
   */
  private void initFromCollection(Collection<? extends E> c) {
    initElementsFromCollection(c);
    heapify();
  }

  /**
   * The maximum size of array to allocate.
   * Some VMs reserve some header words in an array.
   * Attempts to allocate larger arrays may result in
   * OutOfMemoryError: Requested array size exceeds VM limit
   */
  private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

  /**
   * Increases the capacity of the array.
   *
   * @param minCapacity the desired minimum capacity
   */
  private void grow(int minCapacity) {
    int oldCapacity = queue.length;
    // Double size if small; else grow by 50%
    int newCapacity = oldCapacity + ((oldCapacity < 64) ?
        (oldCapacity + 2) :
        (oldCapacity >> 1));
    // overflow-conscious code
    if (newCapacity - MAX_ARRAY_SIZE > 0) {
      newCapacity = hugeCapacity(minCapacity);
    }
    queue = Arrays.copyOf(queue, newCapacity);
  }

  private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
    {
      throw new OutOfMemoryError();
    }
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
  }

  /**
   * Inserts the specified element into this priority queue.
   *
   * @return {@code true} (as specified by {@link Collection#add})
   * @throws ClassCastException if the specified element cannot be compared with elements currently
   * in this priority queue according to the priority queue's ordering
   * @throws NullPointerException if the specified element is null
   */
  public boolean add(E e) {
    return offer(e);
  }

  /**
   * Inserts the specified element into this priority queue.
   *
   * @return {@code true} (as specified by {@link Queue#offer})
   * @throws ClassCastException if the specified element cannot be compared with elements currently
   * in this priority queue according to the priority queue's ordering
   * @throws NullPointerException if the specified element is null
   */
  public boolean offer(E e) {
    if (e == null) {
      throw new NullPointerException();
    }
    modCount++;
    int i = size;
    if (i >= queue.length) {
      grow(i + 1);
    }
    size = i + 1;
    if (i == 0) {
      queue[0] = e;
    } else {
      siftUp(i, e);
    }
    return true;
  }

  @SuppressWarnings("unchecked")
  public E peek() {
    return (size == 0) ? null : (E) queue[0];
  }

  private int indexOf(Object o) {
    if (o != null) {
      for (int i = 0; i < size; i++) {
        if (o.equals(queue[i])) {
          return i;
        }
      }
    }
    return -1;
  }

  /**
   * Removes a single instance of the specified element from this queue,
   * if it is present.  More formally, removes an element {@code e} such
   * that {@code o.equals(e)}, if this queue contains one or more such
   * elements.  Returns {@code true} if and only if this queue contained
   * the specified element (or equivalently, if this queue changed as a
   * result of the call).
   *
   * @param o element to be removed from this queue, if present
   * @return {@code true} if this queue changed as a result of the call
   */
  public boolean remove(Object o) {
    int i = indexOf(o);
    if (i == -1) {
      return false;
    } else {
      removeAt(i);
      return true;
    }
  }

  /**
   * Version of remove using reference equality, not equals.
   * Needed by iterator.remove.
   *
   * @param o element to be removed from this queue, if present
   * @return {@code true} if removed
   */
  boolean removeEq(Object o) {
    for (int i = 0; i < size; i++) {
      if (o == queue[i]) {
        removeAt(i);
        return true;
      }
    }
    return false;
  }

  /**
   * Returns {@code true} if this queue contains the specified element.
   * More formally, returns {@code true} if and only if this queue contains
   * at least one element {@code e} such that {@code o.equals(e)}.
   *
   * @param o object to be checked for containment in this queue
   * @return {@code true} if this queue contains the specified element
   */
  public boolean contains(Object o) {
    return indexOf(o) != -1;
  }

  /**
   * Returns an array containing all of the elements in this queue.
   * The elements are in no particular order.
   *
   * <p>The returned array will be "safe" in that no references to it are
   * maintained by this queue.  (In other words, this method must allocate
   * a new array).  The caller is thus free to modify the returned array.
   *
   * <p>This method acts as bridge between array-based and collection-based
   * APIs.
   *
   * @return an array containing all of the elements in this queue
   */
  public Object[] toArray() {
    return Arrays.copyOf(queue, size);
  }

  /**
   * Returns an array containing all of the elements in this queue; the
   * runtime type of the returned array is that of the specified array.
   * The returned array elements are in no particular order.
   * If the queue fits in the specified array, it is returned therein.
   * Otherwise, a new array is allocated with the runtime type of the
   * specified array and the size of this queue.
   *
   * <p>If the queue fits in the specified array with room to spare
   * (i.e., the array has more elements than the queue), the element in
   * the array immediately following the end of the collection is set to
   * {@code null}.
   *
   * <p>Like the {@link #toArray()} method, this method acts as bridge between
   * array-based and collection-based APIs.  Further, this method allows
   * precise control over the runtime type of the output array, and may,
   * under certain circumstances, be used to save allocation costs.
   *
   * <p>Suppose {@code x} is a queue known to contain only strings.
   * The following code can be used to dump the queue into a newly
   * allocated array of {@code String}:
   *
   * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
   *
   * Note that {@code toArray(new Object[0])} is identical in function to
   * {@code toArray()}.
   *
   * @param a the array into which the elements of the queue are to be stored, if it is big enough;
   * otherwise, a new array of the same runtime type is allocated for this purpose.
   * @return an array containing all of the elements in this queue
   * @throws ArrayStoreException if the runtime type of the specified array is not a supertype of
   * the runtime type of every element in this queue
   * @throws NullPointerException if the specified array is null
   */
  @SuppressWarnings("unchecked")
  public <T> T[] toArray(T[] a) {
    final int size = this.size;
    if (a.length < size)
    // Make a new array of a's runtime type, but my contents:
    {
      return (T[]) Arrays.copyOf(queue, size, a.getClass());
    }
    System.arraycopy(queue, 0, a, 0, size);
    if (a.length > size) {
      a[size] = null;
    }
    return a;
  }

  /**
   * Returns an iterator over the elements in this queue. The iterator
   * does not return the elements in any particular order.
   *
   * @return an iterator over the elements in this queue
   */
  public Iterator<E> iterator() {
    return new Itr();
  }

  private final class Itr implements Iterator<E> {

    /**
     * Index (into queue array) of element to be returned by
     * subsequent call to next.
     */
    private int cursor = 0;

    /**
     * Index of element returned by most recent call to next,
     * unless that element came from the forgetMeNot list.
     * Set to -1 if element is deleted by a call to remove.
     */
    private int lastRet = -1;

    /**
     * A queue of elements that were moved from the unvisited portion of
     * the heap into the visited portion as a result of "unlucky" element
     * removals during the iteration.  (Unlucky element removals are those
     * that require a siftup instead of a siftdown.)  We must visit all of
     * the elements in this list to complete the iteration.  We do this
     * after we've completed the "normal" iteration.
     *
     * We expect that most iterations, even those involving removals,
     * will not need to store elements in this field.
     */
    private ArrayDeque<E> forgetMeNot = null;

    /**
     * Element returned by the most recent call to next iff that
     * element was drawn from the forgetMeNot list.
     */
    private E lastRetElt = null;

    /**
     * The modCount value that the iterator believes that the backing
     * Queue should have.  If this expectation is violated, the iterator
     * has detected concurrent modification.
     */
    private int expectedModCount = modCount;

    public boolean hasNext() {
      return cursor < size ||
          (forgetMeNot != null && !forgetMeNot.isEmpty());
    }

    @SuppressWarnings("unchecked")
    public E next() {
      if (expectedModCount != modCount) {
        throw new ConcurrentModificationException();
      }
      if (cursor < size) {
        return (E) queue[lastRet = cursor++];
      }
      if (forgetMeNot != null) {
        lastRet = -1;
        lastRetElt = forgetMeNot.poll();
        if (lastRetElt != null) {
          return lastRetElt;
        }
      }
      throw new NoSuchElementException();
    }

    public void remove() {
      if (expectedModCount != modCount) {
        throw new ConcurrentModificationException();
      }
      if (lastRet != -1) {
        E moved = PriorityQueue.this.removeAt(lastRet);
        lastRet = -1;
        if (moved == null) {
          cursor--;
        } else {
          if (forgetMeNot == null) {
            forgetMeNot = new ArrayDeque<>();
          }
          forgetMeNot.add(moved);
        }
      } else if (lastRetElt != null) {
        PriorityQueue.this.removeEq(lastRetElt);
        lastRetElt = null;
      } else {
        throw new IllegalStateException();
      }
      expectedModCount = modCount;
    }
  }

  public int size() {
    return size;
  }

  /**
   * Removes all of the elements from this priority queue.
   * The queue will be empty after this call returns.
   */
  public void clear() {
    modCount++;
    for (int i = 0; i < size; i++) {
      queue[i] = null;
    }
    size = 0;
  }

  @SuppressWarnings("unchecked")
  public E poll() {
    if (size == 0) {
      return null;
    }
    int s = --size;
    modCount++;
    E result = (E) queue[0];
    E x = (E) queue[s];
    queue[s] = null;
    if (s != 0) {
      siftDown(0, x);
    }
    return result;
  }

  /**
   * Removes the ith element from queue.
   *
   * Normally this method leaves the elements at up to i-1,
   * inclusive, untouched.  Under these circumstances, it returns
   * null.  Occasionally, in order to maintain the heap invariant,
   * it must swap a later element of the list with one earlier than
   * i.  Under these circumstances, this method returns the element
   * that was previously at the end of the list and is now at some
   * position before i. This fact is used by iterator.remove so as to
   * avoid missing traversing elements.
   */
  @SuppressWarnings("unchecked")
  private E removeAt(int i) {
    // assert i >= 0 && i < size;
    modCount++;
    int s = --size;
    if (s == i) // removed last element
    {
      queue[i] = null;
    } else {
      E moved = (E) queue[s];
      queue[s] = null;
      siftDown(i, moved);
      if (queue[i] == moved) {
        siftUp(i, moved);
        if (queue[i] != moved) {
          return moved;
        }
      }
    }
    return null;
  }

  /**
   * Inserts item x at position k, maintaining heap invariant by
   * promoting x up the tree until it is greater than or equal to
   * its parent, or is the root.
   *
   * To simplify and speed up coercions and comparisons. the
   * Comparable and Comparator versions are separated into different
   * methods that are otherwise identical. (Similarly for siftDown.)
   *
   * @param k the position to fill
   * @param x the item to insert
   */
  private void siftUp(int k, E x) {
    if (comparator != null) {
      siftUpUsingComparator(k, x);
    } else {
      siftUpComparable(k, x);
    }
  }

  @SuppressWarnings("unchecked")
  private void siftUpComparable(int k, E x) {
    Comparable<? super E> key = (Comparable<? super E>) x;
    while (k > 0) {
      int parent = (k - 1) >>> 1;
      Object e = queue[parent];
      if (key.compareTo((E) e) >= 0) {
        break;
      }
      queue[k] = e;
      k = parent;
    }
    queue[k] = key;
  }

  @SuppressWarnings("unchecked")
  private void siftUpUsingComparator(int k, E x) {
    while (k > 0) {
      int parent = (k - 1) >>> 1;
      Object e = queue[parent];
      if (comparator.compare(x, (E) e) >= 0) {
        break;
      }
      queue[k] = e;
      k = parent;
    }
    queue[k] = x;
  }

  /**
   * Inserts item x at position k, maintaining heap invariant by
   * demoting x down the tree repeatedly until it is less than or
   * equal to its children or is a leaf.
   *
   * @param k the position to fill
   * @param x the item to insert
   */
  private void siftDown(int k, E x) {
    if (comparator != null) {
      siftDownUsingComparator(k, x);
    } else {
      siftDownComparable(k, x);
    }
  }

  @SuppressWarnings("unchecked")
  private void siftDownComparable(int k, E x) {
    Comparable<? super E> key = (Comparable<? super E>) x;
    int half = size >>> 1;        // loop while a non-leaf
    while (k < half) {
      int child = (k << 1) + 1; // assume left child is least
      Object c = queue[child];
      int right = child + 1;
      if (right < size &&
          ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0) {
        c = queue[child = right];
      }
      if (key.compareTo((E) c) <= 0) {
        break;
      }
      queue[k] = c;
      k = child;
    }
    queue[k] = key;
  }

  @SuppressWarnings("unchecked")
  private void siftDownUsingComparator(int k, E x) {
    int half = size >>> 1;
    while (k < half) {
      int child = (k << 1) + 1;
      Object c = queue[child];
      int right = child + 1;
      if (right < size &&
          comparator.compare((E) c, (E) queue[right]) > 0) {
        c = queue[child = right];
      }
      if (comparator.compare(x, (E) c) <= 0) {
        break;
      }
      queue[k] = c;
      k = child;
    }
    queue[k] = x;
  }

  /**
   * Establishes the heap invariant (described above) in the entire tree,
   * assuming nothing about the order of the elements prior to the call.
   */
  @SuppressWarnings("unchecked")
  private void heapify() {
    for (int i = (size >>> 1) - 1; i >= 0; i--) {
      siftDown(i, (E) queue[i]);
    }
  }

  /**
   * Returns the comparator used to order the elements in this
   * queue, or {@code null} if this queue is sorted according to
   * the {@linkplain Comparable natural ordering} of its elements.
   *
   * @return the comparator used to order this queue, or {@code null} if this queue is sorted
   * according to the natural ordering of its elements
   */
  public Comparator<? super E> comparator() {
    return comparator;
  }

  /**
   * Saves this queue to a stream (that is, serializes it).
   *
   * @param s the stream
   * @serialData The length of the array backing the instance is emitted (int), followed by all of
   * its elements (each an {@code Object}) in the proper order.
   */
  private void writeObject(java.io.ObjectOutputStream s)
      throws java.io.IOException {
    // Write out element count, and any hidden stuff
    s.defaultWriteObject();

    // Write out array length, for compatibility with 1.5 version
    s.writeInt(Math.max(2, size + 1));

    // Write out all elements in the "proper order".
    for (int i = 0; i < size; i++) {
      s.writeObject(queue[i]);
    }
  }

  /**
   * Reconstitutes the {@code PriorityQueue} instance from a stream
   * (that is, deserializes it).
   *
   * @param s the stream
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    // Read in size, and any hidden stuff
    s.defaultReadObject();

    // Read in (and discard) array length
    s.readInt();

    queue = new Object[size];

    // Read in all elements.
    for (int i = 0; i < size; i++) {
      queue[i] = s.readObject();
    }

    // Elements are guaranteed to be in "proper order", but the
    // spec has never explained what that might be.
    heapify();
  }

  /**
   * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
   * and <em>fail-fast</em> {@link Spliterator} over the elements in this
   * queue.
   *
   * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
   * {@link Spliterator#SUBSIZED}, and {@link Spliterator#NONNULL}.
   * Overriding implementations should document the reporting of additional
   * characteristic values.
   *
   * @return a {@code Spliterator} over the elements in this queue
   * @since 1.8
   */
  public final Spliterator<E> spliterator() {
    return new PriorityQueueSpliterator<E>(this, 0, -1, 0);
  }

  static final class PriorityQueueSpliterator<E> implements Spliterator<E> {

    /*
     * This is very similar to ArrayList Spliterator, except for
     * extra null checks.
     */
    private final PriorityQueue<E> pq;
    private int index;            // current index, modified on advance/split
    private int fence;            // -1 until first use
    private int expectedModCount; // initialized when fence set

    /**
     * Creates new spliterator covering the given range
     */
    PriorityQueueSpliterator(PriorityQueue<E> pq, int origin, int fence,
        int expectedModCount) {
      this.pq = pq;
      this.index = origin;
      this.fence = fence;
      this.expectedModCount = expectedModCount;
    }

    private int getFence() { // initialize fence to size on first use
      int hi;
      if ((hi = fence) < 0) {
        expectedModCount = pq.modCount;
        hi = fence = pq.size;
      }
      return hi;
    }

    public PriorityQueueSpliterator<E> trySplit() {
      int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
      return (lo >= mid) ? null :
          new PriorityQueueSpliterator<E>(pq, lo, index = mid,
              expectedModCount);
    }

    @SuppressWarnings("unchecked")
    public void forEachRemaining(Consumer<? super E> action) {
      int i, hi, mc; // hoist accesses and checks from loop
      PriorityQueue<E> q;
      Object[] a;
      if (action == null) {
        throw new NullPointerException();
      }
      if ((q = pq) != null && (a = q.queue) != null) {
        if ((hi = fence) < 0) {
          mc = q.modCount;
          hi = q.size;
        } else {
          mc = expectedModCount;
        }
        if ((i = index) >= 0 && (index = hi) <= a.length) {
          for (E e; ; ++i) {
            if (i < hi) {
              if ((e = (E) a[i]) == null) // must be CME
              {
                break;
              }
              action.accept(e);
            } else if (q.modCount != mc) {
              break;
            } else {
              return;
            }
          }
        }
      }
      throw new ConcurrentModificationException();
    }

    public boolean tryAdvance(Consumer<? super E> action) {
      if (action == null) {
        throw new NullPointerException();
      }
      int hi = getFence(), lo = index;
      if (lo >= 0 && lo < hi) {
        index = lo + 1;
        @SuppressWarnings("unchecked") E e = (E) pq.queue[lo];
        if (e == null) {
          throw new ConcurrentModificationException();
        }
        action.accept(e);
        if (pq.modCount != expectedModCount) {
          throw new ConcurrentModificationException();
        }
        return true;
      }
      return false;
    }

    public long estimateSize() {
      return (long) (getFence() - index);
    }

    public int characteristics() {
      return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL;
    }
  }
}
